The invention relates to a process for workup of a stream comprising butene and/or butadiene, butane, hydrogen and/or nitrogen, with or without carbon dioxide.
Butenes and butadiene can be prepared, for example by thermal cleavage (steam-cracking) of saturated hydrocarbons, typically proceeding from naphtha as a raw material. The steamcracking of naphtha gives a hydrocarbon mixture of methane, ethane, ethene, acetylene, propane, propene, propyne, allene, methylallene, and C5 and higher hydrocarbons.
A disadvantage of this process for producing butenes and butadiene is that relatively large amounts of unwanted coproducts are inevitably obtained. Alternatively, the butenes can be prepared from butane and butadiene from n-butene, by dehydrogenation.
DE-A 10 2004 059 356 discloses, for example, preparing butadiene by using n-butane as a feedstock. To prepare the butadiene, the n-butane is dehydrogenated in a dehydrogenation zone by nonoxidative catalytic dehydrogenation to give a stream comprising n-butane, 1-butene, 2-butene, butadiene, and hydrogen, with or without carbon dioxide and with or without water vapor. In a second dehydrogenation zone, the 1-butene and 2-butene are dehydrogenated further to butadiene. The stream obtained in the dehydrogenation is subsequently compressed and cooled in order to condense out water. A product stream comprising essentially butadiene is removed by extractive distillation from the residual stream comprising n-butane, butadiene, hydrogen, carbon dioxide and water vapor.
A corresponding process for preparing butadiene from n-butane is additionally also described in DE-A 10 2004 061 514.
A disadvantage of the process described here is that a different solvent than in the extractive distillation is used for the removal of an H2-rich stream in the absorption, and thus a desorption stage for the H2-rich gas is also needed. In addition there is no separation of CO2 and H2.
A further process for workup of a stream comprising butenes is also known from SU-A 1159915. In this process, the stream comprising butenes is subjected first to an absorption and then to an extractive distillation. The solvent used for the absorption and the extractive distillation is acetonitrile. A disadvantage in the case of the use of acetonitrile is that it does not dissolve any carbon dioxide. The use of acetonitrile therefore leads to the effect that the proportion of carbon dioxide in the gas increases since the carbon dioxide is recycled into the butane dehydrogenation together with the hydrogen and is not washed out in the absorption.
WO-A 2006/050969 describes a process for preparing butadiene from n-butane, in which the butadiene-comprising stream from the dehydrogenation is first cooled in order to condense out water. In a further compression stage and cooling, a condensate stream comprising n-butane, butadiene and water is obtained. n-Butane and butadiene are removed from the stream comprising water, n-butane and butadiene, and then separated into a product stream consisting essentially of butadiene and a recycle stream comprising n-butane.
A disadvantage here is that the C4 components are removed from the inert gases by a multistage compression and subsequent condensation. This process stage features a high energy requirement for the compression up to approx. 30 bar. The C4 condensation is effected at a temperature of 10° C., and thus a cooling unit is additionally required.
A process for preparing 1-butene is described, for example, in EP-B 1 682 468. In the process described here, a C4 stream is removed in a two-stage process by absorption and a subsequent desorption of inerts. A disadvantage of the process is that the absorption solvent (tetradecane) is different than the solvent used in the extractive distillation (NMP). Moreover, there is the risk of mixing of the two solvents, which results in a reduced selectivity of the absorption and extractive distillation stages. A further disadvantage here is that the absorbent is selective only for C4 and hence there is no removal of H2 and CO2. The use of different solvents in the absorption step entails a desorption of the C4 component with steam before it is passed on into the NMP extractive distillation column.
It is an object of the present invention to provide a process for workup of a stream comprising butene and/or butadiene, which can be implemented with less complexity and lower costs.